Just came back from Vietnam filled in the tdac before I arrived got an instant reply by email ,obviously done ve a computer not a person.
My only problem was I put Nationality as British overseas territory and the immigration person couldn't find it on their computers took a few minutes and they found my mistake let me through ok told me not to do it again.
My phone has 2 sims Thai and UK, I use lebara £4.50/month for UK just for 2 type verification banking and DTAC immortal sim for Thailand unlimited data for 6 months for upfront payment of 2500bht.
What's app will work off your Thai data but use your UK number.
I came to Thailand on a 6 month tourist visa, I done one run to Cambodia stopped 1 night in Cambodia when returning the Thai immigration commented that I had only stay for 1 night but still let me into Thailand.
After the run to Cambodia I got a retirement visa,much easier.